A Better Way

Nov 01, 2000

This is the second in a two-part series on the growing trend of using trenchless technology to upgrade or replace municipal collection systems. Part I, which appeared in our Oct. '00 issue, described how inflow and infiltration can deteriorate sewage systems. Part II deals with the types of trenchless technology available and their advantages over conventional methods of sewer system installation

Trenchless technology is a process that allows a municipality or industry to install, rehabilitate, replace or repair a sewer line or any other type of utility pipe, with minimum excavation from the ground surface. While trenchless sewer rehabilitation has been used in some form for approximately 25 years, it has just been in the last 10-12 years that technology has improved to the point that it is an accepted practice. It is only recently that it has become the most common method of sewer rehabilitation.

There are several benefits to utilizing trenchless technology. First, this method of sewer rehabilitation is typically less expensive and can be done more quickly than digging and replacing sewer systems. Second, work crews are not forced to dig trenches that can cause street closures and the redirection of traffic; necessitate the repaving of streets and reconstruction of sidewalks; and require the restoration of business and homeowner properties. Third, trenchless technology helps to avoid existing underground utilities. This proves extremely useful in heavy, urban areas where electricity, phone lines and numerous other systems connect underground.

Since its inception, several approaches to trenchless sewer rehabilitation have been taken. The first trenchless sewer rehabilitation was a method commonly known as test and seal. Each pipe joint was pressure tested, and if the joint failed, it was sealed with a chemical grout. The cost was relatively inexpensive, and early individual results were promising. However, upon further review, joints or line segments that were not sealed became weak points in the line, allowing infiltration of water into the line. The grouts used in this early period also deteriorated over time, thus negating their benefit.

Sliplining became the popular trenchless method of rehabilitation in the '80s. The installation procedure is relatively simple. High-density polyethylene (HDPE) is heat fused together to the appropriate length. An insertion pit approximately 20 feet in length is excavated (assuming an eight-foot deep sewer and stable earth) between two manholes within a line segment. The old sewer is broken away at the insertion pit. A HDPE Pipe, which is connected to a cable and a winch, is pulled through the line to the manhole. The procedure is repeated to the other manhole. The pipes are coupled together at the insertion pit. If the private sewer laterals must be reconnected to the rehabilitated main line, the contractor televises the line segment prior to sliplining and marks the services on the ground. Services are reconnected to the HDPE pipe using HDPE saddles or flexible rubber saddles and stainless steel bands.

The downside to sliplined pipe is that it is subject to expansion and contraction with changes in the temperature. An inadequate relaxation period from the time of insertion to the time of sealing the pipe at the manhole can result in the line pulling away from its sealed connection. Proper installation requires a 24-hour relaxation period, proper bedding and backfill at exposed points such as the insertion pit and service reconnections; plus grouting between the annular (or empty) space between the old pipe and the new HDPE. When these procedures are followed, sliplining can be effective. There remains some concern that over time the grout will deteriorate and the annular space could become a conduit for infiltration to enter into manholes.

Today, the two most popular methods of trenchless technology are cured in place and pipebursting. Both evolved from the early trenchless approaches and contractors have turned to these two methods more and more in the last decade.

Cured in place pipelining is implemented by inserting a resin impregnated felt tube into a damaged pipe. The felt tube is expanded against the inner wall of the pipe and allowed to cure. This method of trenchless technology effectively rehabilitates an entire line segment, eliminates the annular (or empty) space between the old pipe and the new liner by forming a bond between the two and provides a durable lining system that effectively prevents infiltration from entering the line segment. This is accomplished without digging a trench and maintains approximately the same size pipe as the original sewer line.

Pipebursting is one of the more recent forms of trenchless technology. A point of entry is prepared at one end of the failed pipe section. A pneumatic pipe-bursting tool with an expander is positioned at the pipe opening. Attached to the head of the tool is a steel line emanating from a constant-tension winch located at the exit point. The tool is launched directly into the failing pipe. The combination of the percussive action of the pneumatic tool and constant tension from the winch enables the tool to effectively burst through the old pipe. The expander pushes and compacts the old pipe fragments and surrounding soil out of the way for the safe installation of the new pipe. Pipebursting allows for an increase in pipe size and for the old pipe to be demolished, thus eliminating the imperfections of the original construction.

Trenchless technology at work
Red Bank, Tenn., is a suburban community near Chattanooga. Its population is approximately 13,000 people and it encompasses a commercial zone to provide services to the area. The city faced two major challenges related to its wastewater treatment system. First, rainfall often caused storm water and wastewater to overflow at several points in the major basin of the collection system, spilling over into residential yards and to the nearby creek, Stringers Branch. Second, Red Bank sought to reduce the cost for the treatment and disposal of its wastewater by eliminating year-round infiltration into the collection system. The city utilized trenchless technology to accomplish both objectives.

The city, with its engineering consultant Gresham, Smith and Partners, burst 55,000 linear feet of concrete and deteriorated clay pipe and rehabilitated approximately 800 service reconnections to area homes and businesses. This effort was designed to reduce inflow and infiltration into the existing 18-inch interceptor sewer (an interceptor sewer serves as a collection or trunk line for other smaller sewers). In addition, Red Bank constructed a 24-inch interceptor parallel to the existing 18-inch pipe to improve the water flow through the entire system.

An interim construction evaluation study for Red Bank was recently completed to determine the effectiveness of the effort. A comparison between past and present flow recordings helped determine the overall success of the work. Similar months were used for comparison including time of year, rainfall, number of rain events in the month and the peak rain event (see Table 1).

The data gathered indicated that Red Bank made tremendous strides in correcting its chronic manhole overflow situation and has reduced total system metered flows by approximately 210,000 gallons per day (gpd)1. The average daily flow recorded in December 1996 was 1.4 million gallons per day (MGD). This is compared to the January 2000 average flow of 1.2 MGD. The decrease is more remarkable when considering that in 1996 a tremendous amount of unquantified overflow spilled from manholes in the system. The December '96 data does not include nearly 50 hours of sewer overflows at various locations. It is impossible to calculate in retrospect how much overflow is not included in the December 1996 average.

Table 1. Comparable data summary

Parameter

December 1996

January 2000

6.22 inches

6.46 inches

No. of rain days

14 days

14 days

Peak rain event

2.84 inches (11/30 & 12/01)

2.53 inches (01/09 & 01/10)

* * Research from each sub-basin (metered area) tabulated separately.

In a seven week period from November 27, 1996 through January 15, 1997, eight flow meters were installed at key manholes to measure the flow of storm and wastewater through the system. In a similar fashion, during an eight month period from August 1999 through March 2000, flow meters were reinstated at four of the eight key manholes to compare pre-construction flows with post-construction flows.

Because Red Bank now has parallel interceptor sewers resulting from the construction of the 24-inch interceptor, two meters, labeled A1 and A2 were installed - one on each interceptor sewer - and compared to the 1996 meter A (see Table 2). The A meters represent 100 percent of Red Banks total system flow. The 1996 meter E is compared to the 1999 meter E (see Table 3). Both installations were in manhole 49 on the 15-inch interceptor. The 1996 meter G is compared to the 1999 meter G (see Table 4). Both installations were in manhole 81 on the 15-inch interceptor. The downstream flow includes flow from within its sub-basin plus the upstream flow from the upper sub-basin(s). As the flow is transported down the interceptor, it has a cumulative effect.

Not depicted in the graphical data is the fact that the Red Bank system did not have any overflows in January 2000. In comparison, on December 1, 1996, manhole 42 overflowed16 hours, manhole 49 overflowed 13 hours, and cleanouts at 2308 and 2408 Lyndon Avenue overflowed 7.5 and 13 hours respectively. If these overflows were calculated in the 1996 figures, they would have considerably raised the average flow.

As the data indicates, the city has seen significant flow reduction since the completion of construction activities. This has helped Red Bank reduce its treatment costs. The city pays $0.68/1,000 gallons to the city of Chattanooga for treatment and disposal of its wastewater. The overall average daily flow reduction reported from the A meters is about 210,000 gpd. This results in an annual savings of approximately $52,000 per year in treatment cost.

Conclusion
As cities and industry seek ways to meet federal and state guidelines regarding sewer rehabilitation, trenchless technology will continue to grow in popularity. It is proving itself a valuable remedy enabling municipalities and companies to effectively meet aging infrastructure challenges.

Table 2. Flow meter A results

Data

December 1996

January 2000

Flow reduction

Peak flow recording

411 MGD2

5.75 MGD

(1.64) MGD

Average of daily peaks

2.09 MGD2

1.83 MGD

0.26 MGD

Average daily flow

1.41 MGD2

1.20 MGD

0.21 MGD

Table 3. Flow meter E results

Data

December 1996

January 2000

Flow reduction

Peak flow recording

4.40 MGD

2.94 MGD

1.46 MGD

Average of daily peaks

1.40 MGD

0.83 MGD

0.57 MGD

Average daily flow

0.98 MGD

0.52 MGD

0.46 MGD

Table 4. Flow meter G results

Data

December 1996

January 2000

Flow reduction

Peak flow recording

3.71 MGD

1.75 MGD

1.96 MGD

Average of daily peaks

0.98 MGD

0.52 MGD

0.46 MGD

Average daily flow

0.67 MGD

0.31 MGD

0.36 MGD

end notes

1 Positive results in individual basins are more dramatic, Because overflow from prior to construction was not metered, the total flow reduction is greater than the metered reduction of approximately 210,000 gpd.

Ricky Oakley, PE is a senior associate and Mike Burgett, PE is an associate with Gresham, Smith and Partners, a full-service engineering and architecture firm in Nashville, Tenn. They can be reached respectively via e-mail at ricky_oakley@gspnet.com and mike_burgett@gspnet.com.

This article originally appeared in the 11/01/2000 issue of Environmental Protection.